Electronic components and interconnect patterns are typically formed on substrates such as semiconductor wafers and flat panel displays by means of projection lithography. As shown in FIG. 1, a projection lithography system typically comprises a radiation source 110, a fixed mask with a mask pattern 120, a substrate stage 150, and a projection system 130. During the projection lithography process, radiation from radiation source 110 is imposed on fixed mask 120. Radiation which passes through fixed mask 120 forms a radiation pattern which is focused onto photoresist coated substrate 140 by projection system 130, thereby forming a reduced image of the mask pattern on the substrate. The radiation pattern may represent the geometric shape of an electrical component or interconnect which is to be formed on the substrate by later processing steps. As shown, projection system 130 comprises an imaging lens disposed between the mask and the substrate for reducing and focusing the transmitted radiation pattern passing through the mask. However, projection system 130 may also include a condenser lens disposed between the radiation source and the mask for directing radiation to the mask as well as filters disposed between the radiation source and the condenser lens and the projection lens and the mask. Substrate 140 is removably fixed to substrate stage 150. A substrate positioning system may be provided to move substrate stage 150 and substrate 140 across the image plane.
Projection lithography operates similarly to typical film developing processes where an image contained on a photographic negative is imposed on photographic paper. In most film developing processes, the image embodied on a photographic negative is enlarged. However, in projection lithography, the mask pattern image is oftentimes reduced by a factor of 2-10× by the imaging lens. When the substrate is a silicon wafer and the radiation pattern represents an integrated circuit structure, the field size of the imaging lens is typically much smaller than the total substrate area which will be patterned and various operational modes, such as a step-and-repeat system, must be employed to pattern the entire substrate area.
Projection lithography is currently the most popular manufacturing technique for high-volume electronics production. However, fixed mask projection lithography systems suffer from numerous deficiencies. Electronic devices comprising multiple layers of features require multiple projection lithography exposures using a different mask for each exposure. Changing masks between exposures requires a significant amount of overhead time as each new mask must be accurately aligned and secured before the projection lithography tool can be brought back on-line for processing, resulting in high manufacturing costs. Additionally, the development of new circuit designs can be impeded by the long lead-time required to obtain prototype masks from a mask manufacturer, and significant product development costs may arise if multiple mask revisions are required. Moreover, manufacturers must purchase, store and maintain a large inventory of masks in order to produce a variety of electronic devices, resulting in high overhead expenses. Furthermore, manufacturing efficiency generally requires that a substrate wafer used in semiconductor chip manufacturing only contain multiple copies of a single semiconductor device. Given these deficiencies and the fact that projection lithography may contribute approximately one-third to one-half of the total cost of semiconductor device manufacturing, significant benefits could be realized if different patterns could be formed on a substrate without requiring a different fixed mask for each pattern.
U.S. Pat. No. 5,045,419 to Okumura discloses a projection lithography system wherein a transmissive liquid crystal display is substituted for a fixed mask. In Okumura, a geometric pattern is formed on the transmissive liquid crystal display by electrically changing the optical contrast of pixels contained in the display. The transmissive liquid crystal display is then exposed to a radiation source, and a radiation pattern is formed by the radiation which passes through the transmissive liquid crystal display. The radiation pattern is subsequently directed to a photoresist coated substrate. The Okumura projection lithography system allows different radiation patterns to be formed on a substrate without the need to change, align, and secure a different mask for each pattern. However, the address electrodes and pixel storage capacitors in a transmissive liquid crystal display may block incident radiation, resulting in reduced radiation throughput for small pixel sizes. This may lead to inconsistencies between the desired geometric pattern and the radiation pattern which is directed onto the photoresist coated substrate. In contrast, reflective liquid crystal displays may be formed with address electrodes and storage capacitors which do not block radiation reflected by the display. Hence, substituting a reflective liquid crystal display for a fixed mask in a projection lithography system may accommodate smaller pixel sizes without degrading the properties of the reflected radiation pattern.
As a result, a need exists for a projection lithography system wherein an electrically configurable reflective liquid crystal display is substituted for a fixed mask.